It is well known to fabricate micro-miniature electronic devices in semiconductor material using conventional photolithographic techniques. The resolution capabilities of such printing is 1 to 2 microns. Moreover, to achieve such resolution, intimate contact may be required between a mask and a semiconductor wafer, which, in time, results in physical damage to the wafer and/or mask.
Higher resolution (sub-micron) pattern definition in device fabrication can be achieved with scanning electron beam lithography. However, a fully versatile electron beam exposure system is an expensive and complex installation. Additionally, in such a system, it is necessary that each pattern of each device be exposed in a sequential point-by-point manner in a program-controlled system. Such a procedure is relatively time consuming and expensive.
Accordingly, it has been proposed that a scanning electron beam be used only to generate high-resolution master masks and replication of the mask patterns onto wafers would then be done in some other manner. Soft X-ray exposure systems have been used to replicate the required sub-micron line widths. However, masks to be used in such systems require that the X-ray transparent portions thereof be extremely thin (e.g., 5 to 10 microns) resulting in fragile and dimensionally unstable masks.
U.S. Pat No. 3,892,973 to Coquin et al. describes a particular mask structure for use in an X-ray lithography system. A thin X-ray transparent film is stretched over and bonded to a support ring and an X-ray absorptive pattern formed thereon. The mask is then positioned proximate a resist-coated wafer and illuminated with X-rays to form a shadow pattern on the resist layer defined by the X-ray absorptive pattern on the thin film. Such a technique has been found to be most effective where only a single mask is required. However, where multiple masking operations at different levels are required, as in the fabrication of a typical integrated circuit, difficulties arise in maintaining registration between mask levels due to dimensional instabilities in each mask.
U.S. Pat. No. 3,742,230 to Spears et al. is directed to a soft X-ray mask having an X-ray opaque support substrate. The support substrate has a plurality of relatively thick webs arranged in a grid fashion to provide support for an X-ray transparent membrane. However, the webs undesirably prevent the exposure of substantial portions of the circuit sites on the semiconductor wafer and such a mask arrangement can be relatively weak in a shear mode when the mask diameter is greater than three inches.
Accordingly, there exists a need for methods and means to expose large diameter wafers to X-ray radiation through a series of masking steps where a number of such masks must register accurately between masking levels while maintaining line width definition in the sub-micron range.